Method of producing thermally stable uranium carbonitrides

ABSTRACT

A thermally stable uranium carbonitride can be produced by adding tungsten and/or molybdenum in the amount of 0.2 wt % or more, preferably 0.5 wt % or more, to a pure uranium carbonitride.

United States Patent Ugajin et a1.

METHOD OF PRODUCING THERMALLY STABLE URANIUM CARBONITRIDES Inventors: Mitsuhiro Ugajin; Ichiro Takahashi,

both of Ibaragi, Japan Assignee: Japan Atomic Energy Research Institute, Tokyo, Japan Filed: Oct. 12, 1972 Appl. No.: 296,851

Forei n Application Priority Data Oct. l6, 1971 Japan 46-81762 US. Cl 423/253; 252/301.1 R; 423/256 Int. Cl C01g 43/00 Field of Search 252/30l.1 R; 423/253, 256

References Cited UNITED STATES PATENTS 8/1965 Webb et al. 423/256 X [451 July 1,1975

Primary Examiner-Richard D. Lovering Assistant Examiner--R. E. Schafer Attorney, Agent, or FirmStevens, Davis, Miller &

Mosher ABSTRACT A thermally stable uranium carbonitride can be produced by adding tungsten and/or molybdenum in the amount of 0.2 wt or more, preferably 0.5 wt or more, to a pure uranium carbonitride.

2 Claims, No Drawings 1 METHOD OF PRODUCING THERMALLY STABLE URANIUM CARBONITRIDES BACKGROUND OF THE INVENTION companied by the release of nitrogen, which results in the formation of liquid uranium metal. The reaction thereof can be expressed as follows:

UN U (liquid) +0.5 N flgas) wherein, UN is uranium nitride which is dissolved in the uranium carbonitride. The existence of metallic uranium is not desirable since it promotes swelling of the uranium carbonitride, a reactor fuel, caused by the irradiation and gives adverse effects to a stainless steel cladding materials.

No attempts have been made nor been reported in the literature to improve the thermal stability of uranium carbonitrides. Theoretically, it is possible to suppress the decomposition of uranium carbonitrides by having applied a higher partial pressure of nitrogen than the decomposition pressure depending on temperature. However, it is believed to be almost impossible, in practice, to do so while a nuclear reactor is operatmg.

Japanese Application No. 69970/1969, as reported by the present inventors, discloses a method of producing a stable uranium carbonitride which does not weaken a stainless steel cladding material. According to the method, uranium dicarbide, a by product which lowers the compatibility with the stainless steel, is fixed as UMeC (wherein Me is W or M0) by the tungsten or molybdenum addition and, therefore, the detrimental effect of uranium dicarbide will be nullified. But the above method has a disadvantage in that the uniformity of dispersion of said fissile material may be decreased and the fuel fissile density, i.e. the density of uranium in the reactor fuel, will be lowered since U (C, N), produced according to the above method, contains a carbide complex, namely UMeC SUMMARY OF THE INVENTION The present inventor has developed a method of producing a uranium carbonitride fuel, a desirable reactor fuel, which does not easily decompose and does not substantially contain UMeC The uranium carbonitride, produced according to the method of this invention, contains neither uranium dicarbide nor uranium metal, and, therefore, the problems as to the compatibility of same with a stainless steel material and the behavior thereof under irradiation are thought to be negligible.

The object of the present invention is, by producing a uranium carbonitride which neither contains uranium dicarbide nor uranium metal and simultaneously strengthening the crystal structure thereof, to remove the thermal instability or a uranium carbonitride, i.e. the thermal decomposition thereof. That is to say, the object of the invention is to provide a method of producing a thermally stable uranium carbonitride by incorporating tungsten or molybdenum in the crystal lattice of the uranium carbonitride. In another aspect, the present invention relates to an improvement of the thermal stability of uranium carbonitrides. The chemical formula of uranium carbonitrides can be expressed as UC N, (O x l) and those compounds are suitable for nuclear reactor fuels. However, since the compounds consist of the solid solution of uranium carbide (UC) and uranium nitride (UN), they still retain the disadvantage, thermal decomposition, that is inherent in uranium nitride. Although said disadvantage may be decreased by dissolving uranium carbide in uranium nitride and, as the result, forming uranium carbonitride, the compound is yet thermally unstable as previously mentioned. Nitrogen, accordingly uranium nitride, cannot be contained in reactor fuels in a large amount from the viewpoint of economy of a neutron flux. In practice, a compound, UC N wherein x is about or less than 0.4 will be admitted as a reactor fuel. Even such a compound as is within the scope of the above composition is, however, recognized to decompose.

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of producing a uranium carbonitride in which a mixture of uranium metal and graphite or uranium carbide (UC) which are added with tungsten and/or molybdenum as a metal-providing component is used as a starting material and the final product, uranium carbonitride, contains elementary metal dissolved in the matrix. Unexpectedly, the present inventors found that the final product obtained according to the present method is very stable for temperature compared with pure uranium carbonitrides of the prior art. The method of this invention comprises the steps of; arc-melting the above materials under nitrogen atmosphere to obtain a uranium carbonitride which neither contains uranium dicarbide nor uranium metal and simultaneously dissolving in a short time tungsten or molybdenum in the uranium carbonitride utilizing the high temperature achieved by the melt of the materials. For the addition of the above metals, the elements or the carbides thereof are conveniently used.

In the preparation of a uranium carbonitride which contains neither uranium dicarbide nor uranium metal, the following relation is found to be established between the composition, temperature of the molten materials (TK) and the partial pressure of nitrogen,

The above formula can be used when a tungsten or a molybdenum component exists in a relatively small amount in the matrix. According to the formula, one

3 skilled in the art can easily determine the partial pressure of nitrogen to apply in practising the method of the present invention.

In the preferred embodiments of the present invention, the melting of the starting materials are effected the arc-melting was finished. After those steps, the at a mp r ur g g rom bOU o mass of the materials placed in the arc-furnace was 3,200k under a partial pressure of nitrogen lower than turned u side down and the above steps were repeated. 1 In one process, the materials were turned upside down For a further detailed explanation Of this invention, Seven times. Various materials were melted in accorthe following examples are given, The examples are dance with the above process, and the results thereof shown as a p f h p f rr m im n s f h are listed in Table 1. Moreover, in order to examine the invention and are not intended to limit the p of the thermal stability of the molten products thus obtained, invention. Modifications and variations thereof can, of the molten Specimens were h d at 1900C f 5 Course, be made Within the technical idea and the hours in a resistance furnace under a vacuum of about scope of this invention. Obviously one skilled in the art 10- mmHg The specimens were Cooled to room eah pp p y modify the exalhpleS accordance perature in about 2 hours. The chemical compositions h the ph 'p and the equipment when he practises and the solid phases of the molten specimens before the mvehtlonand after the vacuum heat treatment are also listed in Table 1, together with the change in composition of the Examples 1 7 specimens. The above results were obtained by metal- The vacuum arc-furnace herein used is a commercial lographic observation through a photomicroscope, available one having a water-cooled tungsten electrode chemical analysis, electron-microprobe analysis and as a cathode and a water-cooled capper hearth as an X-ray analysis. anode. In either example, the amounts of uranium It was revealed-by electron-microprobe analysis that, metal plates, carbon rods and tungsten (or molybdein a molten product in which tungsten or molybdenum num) wire were calculated and weighed to make the are incorporated, either of the metals is dissolved in the total amount thereof about 20 g. The materials were crystal lattice of uranium carbonitride at least up to placed in the arc-furnace. After the furnace was evacuabout 2 weight percent. In Table l, UC N, [W] or ated, nitrogen was admitted into the furnace to give a UC N, [Mo] as denoted defines a uranium carbonidesirable partial pressure therefor (about 0.05 atm.) tride in which tungsten or molybdenum is dissolved. It and a suitable pressure of argon (about 0.65 atm.) was was observed that an excess of tungsten or molybdealso added in order to give a conducting medium for num metal precipitated in a uranium carbonitride maarc current. In such an atmosphere, the materials were trix saturated with tungsten or molybdenum. The kept in the molten state for about 1 minute with a negaamounts of oxygen contained in the molten products tive current of about 250A at a negative voltage of were 0.008 to 0.02 weight percent.

Table 1 4 about 20V after they were preheated with about 90-150A at about 15V for about 30 seconds. Then decreasing the electric power, the materials were heated for seconds, and, finally, the power was cut off and Molten product Chemical composition* Exp. Metal Materials Atmosphere No. addi- (mole ratio) (atm.) Weight Atomic Phase tive Ar (Me) U C Me N N; U C N Me U C N Me 1 l 0.89 0.05 0.7 95.09 4.3 0.61 0.04 50.1 44.5 5.4 UC, ,N ,(x=0.1 2 W 1 0.89 0.005 0.05 0.7 94.52 4.3 0.58 0.6 49.6 44.8 5.2 0.4 UC ,N,[W](x=0. 1) 3 W 1 0.89 0.011 0.05 0.7 94.22 4.2 0.58 1.0 49.6 43.9 5.2 1.3 UC ,N [W](x=0.1) 4 W 1 0.89 0.071 0.05 0.7 90.11 4.07 0.52 5.3 48.3 43.3 4.7 3.7 UC, ,.N,[W](x-'=0.1) W 5 Mo 1 0.89 0.02 0.05 0.7 94.5 4.2 0.6 0.7 49.8 43.9 5.4 0.9 UC, ,N,[Mo](x 0.l) 6 Mo 1 0.89 0.03 0.05 0.7 94.05 4.16 0.59 1.2 49.6 43.5 5.3 1.6 UC, ,N,[Mo](x= 0.1) 7 Mo 1 0.89 0.071 0.05 0.7 92.62 4.24 0.54 2.6 48.3 43.5 4.8 3.4 UC ,N,[Mo](x-" 0.l) Mo Molten product subjected to a vacuum heat treatment Exp. Chemical composition* No.

Weight Atomic Phase A( N/U)** U C N Me U C N Me 1 95.30 4.26 0.44 50.9 45.1 4.0 UC, ,N, U 0.030 2 94.50 4.1 0.6 0.8 50.5 43.5 5.4 0.6 UC ,N,[W] +0003 3 93.72 4.11 0.57 1.6 50.1 43.6 5.2 1.1 UC NAW] 0.002 4 90.05 4.10 0.55 5.3 48.0 43.3 5.0 3.7 UC ,N,[W] W +0.007 5 94.6 4.3 0.6 0.5 49.4 44.6 5.3 0.7 -UC, ,N,[Mo] 0.002 6 94.34 4.1 0.56 1.0 50.2 43.4 5.1 1.3 UC ,N [Mo] 0.006 7 92.57 4.16 0.57 2.7 48.4 43.1 5 3.5 UC, ,N,[Mo] Mo +0.004

*%U 100 (%C +%N %Mc) MN/U) N/U of a specimen after {N/U of a specimcn before the vacuum heat treatment the vacuum heat treatment In Table 1, Example 1 is a comparative one, in which tungsten or molybdenum was not added to the starting materials. However, the molten specimen thereof contains about 0.06 weight percent of tungsten owing to the contamination that comes from the tungsten electrode used herein. The above example discloses that, by subjecting to said vacuum heat treatment, the specimen UC .,N, decomposes which results in containing itself metallic uranium as a decomposition product. The change in chemical composition thereof shows that the production of uranium metal is caused by the vaporization of nitrogen component in the above UC N, preferentially rather than uranium component. In Examples 2 4 and 5 7, tungsten and molybdenum, respectively, were incorporated in the specimens. It was revealed that, in either case, the decomposition of each specimen of UCQN (x 0.1) did not take place by the vacuum heat treatment. That is, the uranium metal which is produced by the decomposition is not detected by photo-microscope observation and electron-microprobe analysis. Moreover, the changes in chemical compositions of those specimens owing to the vacuum heat treatment were within the scope of experimental error. Also the molten specimens of Examples 2 7 did not decompose under similar vacuum heat treatment at 2000 2100C.

As is apparent from Table 1, according to the nitrogen melting method applied in this invention, a part of all of the tungsten or molybdenum in the starting materials was dissolved in an uranium carbonitride matrix simultaneously when the matrix is formed. The uranium carbonitride thus obtained neither thermally decomposes nor contains UMeC and can be used as a stable reactor fuel. The coexistence of tungsten or molybdenum metal as a second phase with the uranium carbonitride does not bring about any disadvantages in stability. Preferably, the addition of these elements should be not so much since too much addition thereof makes the preparation of the uranium carbonitride difficult and the fuel fissile density low.

In addition to the above specimens, the product UC, N, [Me] wherein x is less than about 0.4 (O x 0.4) can be also obtained in accordance with the present invention. The amount of the metal dissolved in the matrix is up to about 3 weight percent.

Moreover, helium may be used, in place of argon, as a conducting medium in the arc-furnace.

What we claim is:

1. A process for producing a thermally stable uranium carbonitride which comprises adding to a material selected from the group consisting of uranium carbide and a mixture of uranium and carbon a metal selected from the group consisting of tungsten, molybdenum and mixtures thereof in an amount of 0.2 to 5.3 weight percent of said metal based upon the produced uranium carbonitride;

melting the resulting mixture under the pressure of nitrogen to insure no yield of uranium dicarbide or uranium metal, and

producing a resulting uranium carbonitride containing at least a part of said metal dissolved in the matrix.

2. The process of claim 1 wherein said carbon is graphite. 

1. A PROCESS FOR PRODUCING A THERMALLY STABLE URANIUM CARBONITRIDE WHICH COMPRISES ADDING TO A MATERIAL SELECTED FROM THE GROUP CONSISTING OF URANIUM CARBIDE AND A MIXTURE OF URANIUM AND CARBON A METAL SELECTED FROM THE GROUP CONSISTING OF TUNGSTEN, MOLYBDENUM AND MIXTURES THEREOF IN AN AMOUNT OF 0.2 TO 5.3 WEIGHT PERCENT OF SAID METAL BASED UPON THE PRODUCED URANIUM CARBONITRIDE, MELTING THE RESULTING MIXTURE UNDER THE PRESSURE OF NITROGEN TO INSURE NO YIELD OF URANIUM DICARBIDE OR URANIUM METAL, AND PRODUCING A RESULTING URANIUM CARBONITRIDE CONTAINING AT LEAST A PART OF SAID METAL DISSOLVED IN THE MATRIX.
 2. The process of claim 1 wherein said carbon is graphite. 